The invention relates to a porous sintered ceramic material based primarily on having crystalline phases of mullite and corundum (alpha alumina) interspersed with an amorphous alumina-silica based phase in proportions that provide a combination of improved strength, creep resistance and dimensional stability at high temperatures of use together with good thermal shock resistance. In a particularly beneficial form of the invention, a honeycomb or cellular monolith structure is made of the material to serve many useful purposes. Especially notable are molten metal filters (e.g. for filtering molten steel and gray iron) and catalytic converters or combustors (e.g. for reducing incompletely combusted emissions from solid fuel burning devices) constructed of such honeycomb structures.
Several porous sintered mullite-corundum ceramic materials have been heretofore proposed and used for a variety of purposes.
U.S. Pat. No. 4,560,478 discloses a porous alumina-based ceramic material with a bulk analytical content of 70-92% by weight of alumina and contains 1-95% by weight of needle-like crystals which apparently are mullite crystals. This material is in the form of a three-dimensional network of strands defining interconnected cells, which is a replica of the polyurethane foam with which it is formed. Dispersed in the network is a nitride, carbide, boride or silicide of metal to provide the resultant body with thermal shock resistance. Such bodies are said to be useful as molten metal filters, traps for particulates in exhaust gases, other filters and catalyst carriers.
Three molten metal filters of porous sintered ceramic honeycomb structure recently found in the commercial marketplace were determined to have the following compositions (% by weight):
______________________________________ Product A B C ______________________________________ Bulk analysis Alumina 69.2 49.2 68 Silica 28.9 48.8 28 Phase analysis Mullite 20 66 19 Corundum 48 -- 49 Cristobalite 9.2 -- 8 Quartz (estimated) 3-8* -- 4-9* Amorphous alumina- 15-20* 34 15-20* silica based phase ______________________________________ (*estimated)
A woodstove catalytic combustor of porous sintered mullite-type ceramic in the form of a honeycomb structure, which has been commercially available, has 72% by weight alumina in its bulk analysis and has a phase analysis, by weight, of about 61% mullite, 26% corundum, 9% cristobalite, and 4% amorphous.
However, our research lead us to the discovery of a porous sintered ceramic material that has the aforementioned combination of improved properties which is not possessed or provided by any of the prior art porous sintered mullite-alumina materials known to us.